Hi,

I would like to know what the influence is of strong magnetic fields on long wires. I want to use a technique for my home system, where I have wires coming from the light switching points to a central place in the fuse boxes.
Those light switches will, when pressed, close a low voltage circuit ( scheme: +5V -> resistor -> switch -> ground). The voltage on the resistor will be connected to and measured by an I2c controlled IO expander (PCF8574).

However, I've been told that this is a 'dangerous' setup. Not dangerous for human beings (since it's all low level voltage), but for the circuitry itself.

For instance, due to strong magnetic fields (eg when there's a thunderstorm), there can be high voltage peaks introduced on those cables, maybe a few hundreds of volts. This is, of course, nefast for the fragile electronics of an IO expander port.

The suggestion was to optically isolate the electronics circuit from the switching circuit.

My question(s):

1. How big is the influence of induction on the wires with the situations given (thunderstorms, lightings,...)?
2. Can those long wires be put in the neighborhood of normal voltage wires (e.g. 220V for Europe)?
3. What can be done to prevent the electronics from being damaged?
4. Is it wise to use twisted pair wires to overcome the 'long' distance (let's say, max. of 15 meter)? For instance, to reduce the influence of other disturbances (e.g. a fan or raiser which is active)?
5. Should those wires be shielded? I'm planing to use UTP for the connecting wires between the switches and the electronic hardware. Should I use FTP or UTP?

I know, a lot of questions, but it would be very helpful if those could be answered.

If needed, I can also give a schematic overview of what I'm trying to say, but I don't know if it's possible to attach files to the subjects (and if it is, how it should be done).

Thanks anyhow in advance,

-- Geert